JP6682269B2 - Method for forming a doped silicon ingot with uniform resistance - Google Patents

Description

  The present invention relates to silicon ingots, and more particularly to methods of forming silicon ingots having uniform resistance.

  The Czochralski method is a technique commonly used to form single crystal silicon ingots. The Czochralski method consists of melting a certain amount of silicon, called the raw material, in a crucible and resolidifying the silicon from the seeds. The seeds oriented with respect to the crystallographic axis of solid silicon are first immersed in a molten silicon bath. Then the seed is slowly pulled upwards. Thus, the solid silicon ingot gradually grows while supplying the raw material to the liquid bath.

  Silicon is typically doped to reduce its electrical resistance. Dopants, such as boron and phosphorus, are compounded in the melted raw material before crystallization or in the raw material before the melting step.

  In the Czochralski pulling method, the dopant tends to accumulate in the molten silicon bath due to the segregation phenomenon. The area of the ingot corresponding to the start of solidification has a lower dopant concentration than the area at the end of solidification.

  That is, the dopant concentration in the silicon ingot gradually increases during crystallization of the silicon ingot. As a result, the electrical resistance changes over the height of the ingot.

  However, it is difficult to use the entire silicon ingot having a variable resistance. For example, fabrication of solar cells requires a certain resistance range. Therefore, it is common practice to scrape one end of the ingot with the highest resistance.

  To improve the conversion efficiency of solar cells, it is intended to form a silicon ingot with a uniform resistance over a significant portion of the height of the ingot.

  Document US 2007/0056504 discloses a technique for forming a silicon ingot with uniform axial resistance while maintaining a constant dopant concentration in the molten silicon bath. Tailoring the resistance is accomplished by adding silicon and dopants to the bath at regular intervals.

  This technique is cumbersome because at each addition step the ingot has to be removed from the bath and wait for the dopant and silicon to completely melt.

  Dopants are added in the form of powder or heavily doped silicon wafers. Under these conditions, the addition of dopants is accompanied by contamination of the silicon with other impurities, especially metallic impurities, which is detrimental for photovoltaic applications. Finally, after pulling up the ingot, if no uniform resistance is achieved, the ingot is either scrapped or recycled.

  Document KR2000021737 describes another technique for making the resistance of a silicon ingot uniform, but in the radial direction rather than the axial direction. This second technique consists of irradiating the ingot with a strong dose of neutrons, which results in doping by elemental conversion. However, doping by element conversion can be performed only in a nuclear reactor. Therefore, it cannot be applied to large scale and low cost.

  There is a need to provide a simple and economical method for forming silicon ingots with uniform electrical resistance and good metallurgical quality.

  This need is created in different areas by providing silicon ingots with varying electrical resistance and containing interstitial oxygen, measuring interstitial oxygen concentration in different (various) areas of the silicon ingot. This is accomplished by calculating the thermal donor concentration to be reached, reaching the electrical resistance target, and subjecting different areas of the silicon ingot to annealing for a predetermined amount of time to form the thermal donor. The annealing temperature in each zone is determined from the thermal donor and interstitial oxygen concentration in that zone and from the annealing time.

  The different zones are preferably distributed over the height of the silicon ingot.

  The method further comprises dicing different areas of the ingot prior to annealing.

  The annealing time is advantageously chosen such that the annealing temperature in different areas of the silicon ingot is between 400 ° C and 500 ° C.

  In a preferred embodiment, the interstitial oxygen concentration is determined by measuring the change in electrical resistance in different areas of the silicon ingot after forming the thermal donor by pre-annealing, and determining the amount of thermal donor formed by the pre-annealing. Subtract from the amount of thermal donors that should be created to determine the annealing temperature in each zone.

Other advantages and features will become more apparent from the following description of particular embodiments, which are given for the purpose of illustration only, without limiting the invention, given below.
FIG. 1 shows the steps of a method for equalizing the electrical resistance of a crystalline silicon ingot. FIG. 2 shows a preferred embodiment of step 2 of FIG. FIG. 3 shows the interstitial oxygen concentration C ₀ of the single crystal silicon ingot with respect to the relative height of the ingot. FIG. 4 shows the relationship between the initial resistance of the ingot of FIG. 3, the annealing temperature to be added to the ingot, and the relative height of the ingot.

(Description of preferred embodiments of the invention)
The technique proposed below consists in correcting the resistance of the silicon ingot by locally forming thermal donors that tend towards the same target resistance in all ingots.

Single crystal silicon obtained by the Czochralski method typically contains 10 ¹⁷ to 2 × 10 ¹⁸ atoms / cm ³ of oxygen, especially interstitial oxygen (that is, oxygen atoms occupying interstitial positions of the crystal lattice). Including.

  However, at temperatures of 350 ° C to 550 ° C, interstitial oxygen forms clusters called double thermal donors (DDT). Each thermal donor DDT produces two free electrons, which causes variations in electrical resistance.

The electrical resistance ρ, in fact, depends on two parameters, the concentration of the majority of the free charge carriers and the mobility μ of these carriers, which depend on the concentration N _DDT of the double thermal donor. The general formula is:
In the formula, q is an elementary charge (q = 1.6 × 10 ⁻¹⁹ C).

  In p-doped silicon, the majority of free charge carriers (holes) is determined by the amount of acceptor-type dopant impurities, such as boron atoms (B), embedded in the silicon. In that case, m = [B].

  On the other hand, in n-type silicon, the number of free charge carriers (electrons) is determined by the amount of donor-type dopant impurities such as phosphorus atoms (P). In that case, m = [P].

After thermal treatment, each thermal donor emits two electrons, the "double" character of the thermal donor. The free charge carrier concentration is modified in the following manner: for n-type silicon:
And with p-type silicon,
Is.

Therefore, after formation of the thermal donor DDT, the electron concentration is increased by doubling the N _DDT concentration with respect to the n-type silicon. In p-type silicon, hole concentration is reduced by doubling the N _DDT concentration following charge rebalancing.

  Mobility μ refers to the ability of a charge carrier to displace in a material by the action of an electric field. Electron and hole mobilities in silicon depend on the temperature of the material (T ′) and the concentration of donor and / or acceptor type dopants.

Considering the thermal donor DDT (which is a donor type dopant), the mobility μ is represented by the following relational expression.
T _n is the temperature of silicon normalized to the ambient temperature (T _n = T ′ / 300). N _{A / D} is the ionized acceptor dopant impurity concentration N _A and / or the donor dopant impurity concentration N _D (eg, boron and phosphorus). The parameters μ _max , μ _min , N _ref , α, β1, β2, β3, β4 are given in the table below for the two types of charge carriers in silicon.

The above relational expressions (1) to (4) represent the dependence between the electrical resistance ρ of silicon and the concentration N _DDT of the double thermal donor generated by the heat treatment or annealing of silicon.

  Therefore, it is proposed to apply the thermal donor formation phenomenon to the correction of the electrical resistance of the silicon ingot.

  FIG. 1 shows steps F1 to F5 of the method for obtaining a silicon ingot having an almost uniform electrical resistance.

  In the first step F1, a crystalline silicon ingot whose electric resistance fluctuates is prepared. Crystalline silicon ingots have a minimum vertical dimension, or height, of about 1 centimeter to a few meters. The ingot is preferably obtained from the molten silicon bath by the Czochralski (Cz single crystal silicon) pulling method. A silicon ingot obtained by directional solidification can also be used. This type of silicon, in fact, contains oxygen necessary for the formation of thermal donors, similar to Cz single crystal silicon.

  In addition to oxygen, the silicon of the ingot may contain dopants such as boron and / or phosphorus. These dopants are added to the molten silicon feedstock prior to pulling the ingot, or are present in the feedstock from the beginning, ie prior to the melting step. When crystallization is complete, the dopant is non-uniformly distributed in the ingot, which leads to large variations in electrical resistance, for example up to a factor of 10.

  Silicon ingots crystallized from undoped charges can also be used. In that case, the variation in electrical resistance on the scale of the ingot is due to the residual dopants that were not removed during the silicon refining process, or due to the thermal donors formed during crystallization (but their amount). Is not the result of the rest of the method). The electrical resistance is initially high (typically 100 Ω.cm to 1000 Ω.cm), but subsequent formation of thermal donors reduces this resistance due to the generation of free electrons. Doping the silicon with dopants is thus avoided, which limits the contamination of the silicon with carbon or metallic elements.

  Step F2 consists of measuring the interstitial oxygen concentration in different areas of the silicon ingot. These areas are preferably distributed over the height of the ingot. Height is defined as the dimension of the ingot along the axis at which the silicon solidifies. Therefore, this preferred embodiment seeks to obtain a uniform axial electrical resistance.

The interstitial oxygen concentration (hereinafter referred to as C ₀ ) can be measured by Fourier transform infrared spectroscopy (whole rod FTIR) over the entire height of the ingot. This technique makes it possible to measure the absorption of infrared radiation in silicon for the wavelength of this radiation. However, interstitial oxygen contributes to this absorption. Therefore, the concentration C ₀ can be estimated from the absorption measurement.

A second technique, also based on the formation of thermal donors, allows the oxygen concentration C _{0 in} silicon to be measured. This technique is described in detail in French patent application FR 1003510 for performing oxygen mapping of silicon wafers. Here we apply in an advantageous manner on an ingot scale.

FIG. 2 details this preferred embodiment. The step F2 of measuring the concentration C ₀ can be divided into several sub-steps F20 to F24.

  At F20, the initial electrical resistance at ambient temperature is measured in each zone of the silicon ingot.

  Then, in a sub-process F21, the silicon ingot is subjected to a preliminary heat treatment to form a double thermal donor (DDT). Unlike the second anneal, which is designed to homogenize the electrical resistance of the ingot, the temperature of this first anneal is constant in the ingot. This temperature is preferably 350 ° C to 500 ° C.

  After this annealing, the electrical resistance is measured in each zone of the ingot at the ambient temperature by, for example, the four-point method (Van der Pauw method) (quasi-process F22).

Since the resistance variation is due to the formation of the thermal donor, the concentration N _DDT ′ of the thermal donor formed by this preliminary annealing is estimated therefrom in the sub-step F23. Relational expression (1) is used for this purpose.

Finally, in step F24, the oxygen concentration C ₀ of each measurement zone is determined from the concentration N _DDT ′ and from the pre-annealing time. It is advantageous to use a chart that gives the concentration C ₀ for different annealing times and temperatures.

In step F3 of FIG. 1, the concentration of the double thermal donor N _DDT to be created in each zone of the ingot in order to reach the target value of electrical resistance ρ _T is calculated. Since the initial resistance varies in the ingot, the amount of thermal donor to be generated is not the same depending on the height of the ingot.

This calculation uses the above relational expressions (1) to (4) that connect the resistance ρ to the concentration N _DDT of the double thermal donor (replace ρ (N _DDT ) by ρ _{T in the} relational expression (1)). Further knowledge of the dopant impurity concentrations N _A and N _D in different areas of the ingot is needed. If these concentrations are not known (the ingot manufacturer has generally established a doping profile throughout the ingot), these concentrations are determined in a preliminary step, for example by measuring the initial resistance in each zone. can do.

The target value ρ _{T of the} resistance is selected according to the intended use of the silicon ingot. For example, in the production of solar cells, 0.5Ω. cm to 10 Ω. cm.

Upon completion of step F3, an interstitial oxygen concentration value C ₀ and a thermal donor concentration value N _DDT are obtained for each zone of the silicon ingot.

Step F4 consists of calculating the annealing temperature T required to obtain the thermal donor concentration N _DDT calculated in step F3, for a predetermined annealing time t. The mathematical model obtained from the paper ["Unified model for formation kinetics of oxygen thermal donors in silicon"; K. Wada, Physical Review B, Vol. 30, No. 10, 1984] is used here. This paper describes the relationship between the thermal donor formation kinetics and the annealing temperature T. This mathematical model is as follows.
Where:
_-C0 is the interstitial oxygen concentration.
• t is the annealing time.
A and B are constants determined by a person skilled in the art, in particular about 5.6 × 10 ⁻⁶ and 5.1 × 10 ⁻⁵ , respectively, more specifically 5.6 × 10 ⁻⁶ and 5. It is 1 × 10 ⁻⁵ .
-N is the electron content at the annealing temperature, and for n-type silicon
Equal to
With p-type silicon
Equally, n _i is the intrinsic carrier concentration in silicon is given by the equation below,
In the formula, k _B represents a Boltzmann constant.
D _i is an interstitial oxygen diffusion coefficient and is represented by the following equation.
Eg is the bandgap energy according to the annealing temperature T (represented by K).

Relational expression (5) links the thermal donor concentration N _DDT to the annealing temperature T, the interstitial oxygen concentration C _0, and the annealing time t. Therefore, the annealing time t expressed in seconds is expressed by the following relational expression.

Since the concentrations C ₀ and N _DDT are known, a suitable annealing time t is simply chosen to obtain the annealing temperature T in each measurement zone according to the relation (5). The time t is the same for all zones of the ingot as each zone is heat treated together. The time t is chosen such that the annealing temperature T in the different zones does not exceed 550 ° C. Above this temperature, thermal donors are not formed. On the contrary, the thermal donor begins to decompose.

  If the temperature T calculated in step F4 exceeds 550 ° C., the time t increases and the temperature value T calculated for the different zones decreases to the temperature range of 350-550 ° C. The value of the annealing time t is preferably chosen such that the temperatures of the different zones are in the temperature range 400 ° C. to 500 ° C. where the above model is most accurate.

If the oxygen concentration C ₀ is determined from the resistance variation after pre-annealing (FIG. 2), thermal donors have already occurred. The calculation of the temperature T of the second anneal must take this into account (otherwise, another anneal at temperatures above 650 ° C., ideally for 1 hour, should eliminate these donors. Must be). In that case, in order to calculate the temperature T, the concentration N _DDT ′ (measured in step F23) of the thermal donor generated by the first annealing is subtracted from the concentration N _DDT (measured in step F3). The calculation of the amount of the thermal donor remaining for creating is originally performed for each measurement area of the ingot.

  After the various temperatures have been calculated, the silicon ingot undergoes a final anneal in step F5 (FIG. 1) to obtain an almost uniform resistance. A temperature gradient is applied between different zones of the ingot such that each zone is at its temperature T.

  In a preferred embodiment, different areas of the ingot are diced prior to final annealing. The ingot is diced into, for example, 4 or 5 parts having a height of about 20 to 40 cm. The furnace used for annealing comprises several temperature zones, each part of the ingot being placed in a zone of a specific temperature.

In a special aspect, the silicon ingot provided in step F1 can comprise a radially homogeneous oxygen atom concentration C ₀ . Therefore, the ingot produced by the method is advantageously homogeneous in resistance both longitudinally and radially. Further, the method may include one or more additional steps to homogenize the oxygen atom concentration C ₀ in the silicon ingot in the radial direction.

  The method for correcting the electrical resistance of the ingot can be applied to silicon doping type, p-type or n-type. In the case of n-type doping, the formation of thermal donors increases the number of majority charge carriers, or electrons. As a result, the resistance is reduced. On the other hand, in the case of p-type doping, the electrons generated by the thermal donor cancel the holes of silicon, so that the resistance increases.

  Nevertheless, weak (and constant) electrical resistance is obtained by heavily doping silicon prior to silicon crystallization. Ingots with weak and uniform resistance can be advantageously used to make solar cells. In an advantageous manner, this method can be used to produce silicon ingots and by dicing the ingots, 1-10 Ω. Wafers with a resistance of cm can be produced.

  In addition, p-doping can be converted to n-doping by creating more electrons than there were initially holes in the ingot and by keeping the resistance of such an ingot constant.

  It should be noted that in n-doped silicon, at ambient temperature, some of the thermal donors formed by annealing are single thermal donors. Single thermal donors generate a single electron instead of two for double thermal donors.

The amount of the single thermal donor alone can be ignored when the concentration m is less than 5 × 10 ¹⁵ cm ⁻³ . In that case, it can be estimated that all of the thermal donors are double, and the relational expressions (1), (2) and (4) can be applied.

On the other hand, if the concentration m is greater than 5 × 10 ¹⁵ cm ⁻³ , then the thermal donor concentration (step F3) is calculated, preferably taking these single thermal donors into account, and then the annealing temperature of each zone is calculated (step F4).

On the other hand, the relational expression (2) that gives the free charge carrier concentration m is modified as follows.
_Where N _SDT is the concentration of a single thermal donor.

On the other hand, the relational expression (4) of the mobility μ is also corrected.

Calculation of the thermal donor concentration formed when performing the annealing step will now include two unknowns, the double thermal donor concentration N _DDT and the single thermal donor concentration N _SDT (relationship 1 → ρ (N _DDT , N _SDT ) = Ρ _T ). Therefore, a second equation linking the unknowns N _DDT and N _SDT is needed to determine the latter.

The second equation is given by the ratio of the concentrations N _DDT and N _SDT calculated at the ambient temperature T _a . This ratio is described below.
Where E ₂ is the deep energy level introduced by the thermal donor (at 150 meV below the conduction band) and E _F is the Fermi level.
And k _B is the Boltzmann constant.

By solving the system of equations (1), (2 ′), (4 ′) and (4 ″), both the single thermal donor concentration N _SDT and the double thermal donor concentration N _DDT are determined. The total concentration of thermal donors _NDT = N _SDT + N _DDT to be formed during annealing to reach ρ _T is estimated therefrom.

The total concentration _NDT is then used to calculate the temperature in step F4. In other words, in equation (5), the double thermal donor concentration N _DDT is replaced by the total concentration N _DT calculated in advance.

  As indicated above, the difference between the final electrical resistance of the ingot and the initially set resistance target value is minimized, taking into account the single thermal donor. This improves the accuracy of the method.

  The method according to FIG. 1 and its alternative embodiments are preferably carried out over the entire height of the silicon ingot. In this way, scrap products are avoided. Unlike prior art methods, it is possible to recycle discarded ingots by correcting their resistance after solidification is complete.

3 and 4 illustrate exemplary aspects of the method of FIG. Ingot to be used by the Czochralski method, it was crystallized from silicon raw material containing phosphorus ^{_{(N D = [P] =}} 10 15 cm 3). Such ingots are standard in the photovoltaic industry.

Axial resistance is 1Ω. cm (ρ _T = 1Ω.cm), and it is desirable to obtain an ingot in which the annealing time t is fixed at 1 hour (the shorter the time, the higher the annealing temperature).

FIG. 3 shows the concentration C ₀ measured over the height of the ingot. Here the height is calculated relative to the end of the ingot which corresponds to the start of solidification and is expressed as a percentage of the total height (relative height).

FIG. 4 shows the relationship between the initial electrical resistance of the silicon ingot (left y-axis) and the relative height of the ingot. The target resistance ρ _T is shown by a broken line. The electrical resistance is 2Ω. cm to about 12 Ω. It is observed to vary between cm.

  Using this plot and the relations (1) to (4), the resistance is set to 1Ω. It is possible to calculate the amount of thermal donor that should be created to bring down to cm.

By knowing the thermal donor concentration N _DDT and the oxygen concentration C ₀ , the annealing temperature profile T to be added to the ingot can be determined (relational expression (5)). This profile is an example of a phosphorus-doped ingot and is also shown in FIG. 4 (right y-axis).

  The silicon ingot (or part of the ingot, if applicable) obtained by this method can advantageously be sliced into silicon wafers and used to form solar cells. The long life of charge carriers in these wafers makes them particularly suitable for advanced structures in solar cells, eg heterojunction cells.

  The life of charge carriers is longer the less metallic impurities of silicon (iron, nickel, copper, etc.). This is achieved by forming thermal donors inside the material, rather than by adding the dopant "in situ", ie in the form of a strongly doped wafer or powder.

Of course, the greater the number of measurement areas in the ingot, the higher the accuracy of the oxygen concentration C ₀ (FIG. 3), thermal donor concentration N _DDT and temperature T (FIG. 4) measurements over the ingot height. After the method of FIG. 1 is completed, a nearly flat resistance profile is obtained at the target resistance ρ _T level.

The relations (1) to (4) can be generalized to so-called offset silicon, which simultaneously provides all types of doping, in particular both types of dopants, acceptors and donors. The initial free charge carrier concentration is equal to the difference (in absolute value) in the concentration of the dopant impurities of the acceptor type N _A and the donor type N _D , whereas the parameter N _{A / D} in equation (3) is Equal to the sum of the concentrations (N _A + N _D ).

When we are in the presence of acceptor dopants of different nature (for example boron and gallium), the concentration N _A is equal to the sum of the concentrations of these dopants (if necessary manipulated by their respective ionization factors). To). The same applies to the concentration N _D for multiple donor dopants (excluding thermal donors).

Claims (7)

A method of forming a silicon ingot, comprising:
A step (F1) of preparing a silicon ingot having varying electrical resistance (ρ) and containing interstitial oxygen,
Measuring the interstitial oxygen concentration (C ₀ ) in a plurality of different zones of the silicon ingot (F2),
Calculating the thermal donor concentration ( _NDDT , _NDT ) of each of the different zones of the silicon ingot to be created in each zone so that the electrical resistance value in each zone of the silicon ingot is uniform. And a step (F3) in which the electric resistance values in the respective areas are substantially the same and the calculation is made within the range of 0.5 Ωcm to 10 Ωcm.
- the silicon ingot, comprising the steps of: determining an annealing temperature (T) at a plurality of different zones (F4), the concentration of thermal donors _{(N DDT,} N _DT) of said zone and an interstitial oxygen concentration _{(C 0} ), And from the annealing time, each zone of the plurality of different zones of the silicon ingot is annealed to form the thermal donor and the electrical resistance of each zone of the silicon ingot is determined. A homogenizing step (F5), wherein for each zone of the plurality of different zones, the annealing is determined by the annealing temperature and the annealing time.
Method of forming a silicon ingot.   The method of claim 1, wherein the plurality of different zones are distributed throughout the height of the ingot. The method of claim 1 or 2, comprising the step of dicing the different areas of the silicon ingot prior to the step of annealing the different areas of the silicon ingot (F5). The annealing time (t) is expressed by the following relational expression.
Determined in the formula,
_-NDDT is the calculated thermal donor concentration,
-C ₀ is the interstitial oxygen concentration,
-N is the electron content at the annealing temperature,
A and B are constants determined by a person skilled in the art, in particular about 5.6 × 10 ⁻⁶ and 5.1 × 10 ⁻⁵ , respectively, more particularly 5.6 × 10 ⁻⁶ and 5. 1 × 10 ⁻⁵ ,
-The method according to any one of claims 1 to 3, wherein D _i is the diffusion coefficient of the interstitial oxygen.   Method according to any one of claims 1 to 4, wherein the annealing time (t) is selected such that the annealing temperature (T) in the different zones of the silicon ingot is between 400 ° C and 500 ° C. . The interstitial oxygen concentration (C ₀ ) is determined by measuring the change in electrical resistance (ρ) in the different areas of the silicon ingot after forming a thermal donor by pre-annealing, and forming by the pre-annealing. 6. The concentration of thermal donor created is subtracted from the concentration of thermal donor to be created to determine the annealing temperature (T) in each zone of the different zones, according to any one of claims 1-5. Method.   The method according to claim 1, wherein the interstitial oxygen concentration is measured by Fourier transform infrared spectroscopy.